The mountings of wheels on vehicles have, as a rule, one seal subjected to mostly high load due to dirt and due to other influences of the external surroundings and, axially on the other side of the mounting, one seal subjected to less load. On account of the different loads on the seal caused by the environment, the seal subjected to higher load therefore, as a rule, as what is known as a cartridge seal, with at least two sealing lips, seals off the interior of the wheel mounting in a multiple way, leaktight, inwardly with respect to the external surroundings.
The sealing lips of the seals are prestressed elastically against a sealing surface. The sealing surfaces are cylindrical surface areas (inner cylindrical, preferably outer cylindrical), the axis of symmetry of which is the axis of rotation of the bearing. The sealing lips run on the sealing surface about the axis of symmetry in the direction of rotation in relation to the sealing surface. It is possible that the seal is fixed in relation to the surroundings and the sealing surface rotates with respect to the sealing lip or that the sealing surface is fixed in relation to the surroundings and the sealing lip runs on the sealing surface about the axis of symmetry in the circumferential direction. In both instances, friction arises during contact between the sealing surface and the contact zone of the sealing lip with the sealing surface. The intensity of the friction depends on many different factors, Mention is made here, as examples, of the relative speeds, the materials and the structures of the surfaces of the seal/frictional-surface friction partners and lubrication during frictional contact.
The prestress with which the seal is prestressed against the sealing surface has an important influence on the frictional behavior. The prestress is determined by tolerances and by the function of the sealing lip and is preselected correspondingly, but also as a function of influences, described below, which occur when the seal is in operation.
The seals, as a rule, are subjected to load due to pressure drops between the bearing interior and the external surroundings. Depending on the design of the seal and on the operating conditions, the bearing interior is loaded with underpressures or overpressures, as compared with the pressure in the external surroundings. Temperature rises, for example during driving, lead to overpressures in the inner space of the bearing. When the bearing is at a standstill, the bearing interior cools down again to the temperature of the external surroundings, underpressure occurring in the bearing interior. Both overpressures and underpressures have disadvantageous effect on the operating behavior of the mounting or of the seals and therefore have to be compensated.
The sealing lips of the seals are designed, as a rule, such that they block against pressure in the axial direction and yield in a directed way to overpressures or underpressures in the other direction. The primary task of the seals and therefore of the sealing lips is to protect the interior of the wheel mounting against influences arising from the external surroundings by sealing of the bearing interior with respect to the influences, such as dirt and water, from outside. Overpressure on the outside because of underpressure in the interior of the bearing presses the sealing lips onto the sealing surface to a greater extent and increases the sealing action, that is to say, in this case, the sealing lips also block inwardly against overpressure from outside. The underpressure in the bearing interior cannot be compensated by means of these inward-blocking sealing lips. On account of the higher pressing force on the sealing lips from outside, the above-mentioned radial prestress and, consequently, the friction between the seal and sealing surfaces increases. This results in undesirable wear and higher operating temperatures.
However, inward-blocking sealing lips compensate when there is a pressure drop from the inside outward. Overpressures from inside lift off the sealing lips in sealing contact from the sealing surfaces elastically. Overpressures in the bearing interior are thus compensated outwardly.
The bearing interior, as a rule, is greased. At high temperatures, the grease becomes liquid and possibly, during pressure compensation, escapes outward via the above-mentioned sealing lips. This may lead to a deficient lubrication of the rolling bearing. It is therefore expedient to provide seals with at least one sealing lip which holds the grease in the bearing in the event of the compensation of overpressures from the interior of the bearing outward. Sealing lips of this type block the bearing interior against the emergence of grease from the inside outward, but also, in the event of overpressures in the bearing interior, against pressure compensation from the inside outward. However, underpressure prevailing in the bearing interior can be compensated via these outward-blocking sealing lips, in that these sealing lips lift off from the sealing surface elastically on account of overpressure in the external surroundings.
As mentioned initially, the primary task of the cartridge seals is, on that side of the wheel mounting which is subjected to a high load due to environmental influences, to block the bearing interior inwardly against the influences from outside. The introduction of a device for the compensation of pressure drops from the outside inward needlessly increases the cost of the cassette seal which already has a complex construction. Therefore, it lends itself to integrate an arrangement for the compensation of underpressures in the bearing interior into the seal which is subjected to less load and therefore has a simpler design.
A seal of this type is described in U.S. Pat. No. 4,844,480. The seal of the generic type has one or more diaphragm orifices as ventilation orifices on the inward-blocking first sealing lip lying axially on the outside in the sealing arrangement. The ventilation orifices provide, in the sealing lips blocking further inward against dirt and water, an axial passage for pressure compensation into an annular prechamber of the seal. However, because of the small size of the orifices, dirt can scarcely enter the prechamber. The second seal follows the first sealing lip axially toward the bearing interior and blocks against the emergence of grease outward to the prechamber. Overpressures in the prechamber act on the second sealing lip until the latter lifts off and the underpressure in the bearing inner space is compensated.
A seal of this type is unsuitable or is suitable to only a limited extent for use on wheel mountings of vehicles, since the diaphragm orifices may become clogged with dirt during driving and the functioning of the seal therefore is adversely influenced.